Explanation

(c) Let v be the actual frequency of sound of horn.
If $v_{s}$be the velocity of car, then frequency of sound striking the cliff
$v^{'} = \frac{v \times v}{v - v_{s}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ldots(i)$
The frequency of sound heard on reflection
$v^{''} = \frac{(v + v)v^{'}}{v} = \frac{\left( v + v_{s} \right)}{v} \times \frac{v \times n}{\left( v - v_{s} \right)}$
Or
$\frac{v^{''}}{v} = \frac{v + v_{s}}{v - v_{s}} = 2$
$v + v_{s} = 2v - 2v_{s}\ $
$\therefore 3v_{s} = v$
$or\ $
$v_{s} = \frac{v}{3}$